Prior to the trade, Baseball America ranked the 5-foot-11 right-hander as the Athletics’ 12th-best prospect.

Following the change, it took Derby a while to find his footing in the Brewers’ system. In what appears to be his final season as a full-time starter, Derby posted a 6-13 record with a 5.59 ERA for High-A Brevard County.

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The Brewers moved Derby, a former sixth-round selection out of San Diego State University, to the pen and jumped him a level to the Biloxi Shuckers this season. The change has done wonders for his production thus far as his numbers have practically all improved across the board. Heading into Wednesday’s series opener at the Tennessee Smokies, he boasts a 2-0 record with a 1.72 ERA. He’s also holding batters to a paltry .187 average, much better than the .306 clip last season.

“Last year, my mentality was try and throw your fastball as hard as you can,” Derby said before Monday’s game against the Chattanooga Lookouts. “It didn’t really pay off because when I really try and let it go I can get out of my mechanics a little bit. That’s not something I want to do because when I do that, I’ll over-stride and then my fastball flattens out and it just makes it a lot easier for them to hit.”

The call he was moving to the pen stung, he said, but he wasn’t completely surprised. After 311/3 innings, it’s hard to argue with the results.

“Once I was drafted in 2015 I was honest with myself — and that’s the best thing you can do – and told myself my stuff plays better out of the pen and if I’m going to have a chance to go anywhere it’ll be as a bullpen guy,” he said.

Derby mixes a fastball, slider and changeup, and has added a curveball to his repertoire. Baseball America previously ranked his changeup as the best secondary pitch in the A’s system.

“That’s the pitch I lost a little bit of a feel for last year,” he said. “This year, I worked really hard to try and get that back. I think a good fastball-changeup mix is one of the best combos you can have.”

As a low-90s guy, Derby relies mostly on his command and movement. He currently boasts 28 strikeouts and 11 walks (including three intentional free passes).

“I’m not a guy who’s able to throw 95 miles per hour and can blow it by you at any given moment,” he said. “It’s a sinker mentality and I’m just trying to get early outs. I try to trust my two-seam fastball to sink in, two-seam changeup, curveball, slider.”

“There’s a full commitment and conviction to a pitch,” he said. “Outing after outing there’s a commitment to a pitch and whatever happens he makes the next pitch. That’s what everyone should be doing.

“When he makes a mistake it hasn’t been the hitter’s mistake, it’s been his mistake. Might pull off of a ball but they can’t do anything with it and then he comes back to make the next pitch.”

Hook called Derby a “nice utility knife for us,” and it’s easy to see why. Although he doesn’t have a save on the season, he’s been used in plenty of situations. He threw 31/3 innings in his last relief appearance. Three weeks before, he pitched 5.0 scoreless. In between are plenty of shorter outings as well.

“He has done a good job of giving us multiple innings at the back end of a game,” Hook said. “He has been comfortable there and not everybody is comfortable at the back end of games.”

Derby’s having a memorable season thus far, but he’s not resting on his early success.

“My mentality has always been never satisfied,” he said. “Things are going great right now and I’m happy with it, but I try not to think about the numbers too much.